1. Field of the Invention
The present invention relates to a capillary electrophoretic instrument having a capillary array assembly comprising a plurality of capillaries and, more particularly, to a capillary electrophoretic instrument (hereinafter referred to as an electrophoretic instrument) suitable for use in such as a DNA sequencer (a DNA base sequence analyzer) for analyzing samples of living organism, especially through the use of a plurality of capillaries or minute passages as a medium of electrophoretic separation, and to a capillary array assembly used in the instrument.
2. Description of the Prior Art
In the base sequence determination of DNA having a very long base sequence, a shift is occurring from a conventional flat-plate gel method, in which a gel is sandwiched between two glass plates and DNA, which is a sample, is caused to migrate electrophoretically by applying a voltage across both ends of the glass plates, to a capillary electrophoretic instrument, in which a gel is filled in capillaries made of quartz (hereinafter referred to as capillaries) and a sample is caused to migrate electrophoretically by applying a voltage across both ends of the capillary array assembly.
The above capillary electrophoretic instrument, which permits high-speed and high-sensitivity analyses in comparison with the flat-plate gel method and is less affected by the Joule heat from self-heat generation by a migration current, can provide a good resolution for an electrophoretic analysis.
In recent years, in order to increase the number of analyses per unit time allowed in one electrophoretic instrument, electrophoretic instruments in which a multiple of capillaries are set and the DNA analyses of a multiple of samples can be simultaneously performed have been coming into widespread use.
In many of these instruments, as a method for applying a voltage to the sample loading side of each capillary during sample loading into capillaries or during electrophoresis, a sample plate in which samples are set and a buffer tank for electrophoresis them selves are made of a conductor, such as a metal, or electrodes are embedded in the sample plate and buffer tank.
As in the art described in JP-A-10-206382, there is also a method in which an electrophoretic instrument has such an electrode structure that an electrode covers the area surrounding the sample loading portion of each capillary and electrophoresis is performed by applying a high voltage to the electrophoretic instrument via a wiring pattern connected to each electrode.
In the above-mentioned technique in which the sample plate and buffer tank for electrophoresis them selves are made of a conductor such as a metal, an analyst must have ready a large number of sample plates having a voltage application structure peculiar to each DNA analyzer as mentioned above for the NDA analyses of a large number of samples. This has posed the problems of increased running costs related to analyses and increased burden on analysts.
Next, it is desirable that a general-purpose microtiter-plate, etc. is capable of being used in an electrophoretic instrument. Of course, however, this microtiter-plate is not provided with an electrode portion capable of being connected to the electrophoretic instrument. For this reason, a technique for incorporating electrodes in an electrophoretic instrument cannot be used.
Furthermore, in an electrophoretic instrument which has an electrode structure portion covering the area surrounding the sample loading portion of each capillary and is provided with a wiring pattern connected to each electrode structure portion and to which a high voltage is applied, the capillary replacement work is very troublesome.
In addition, capillaries have a short life and in some applications it is necessary to perform analyses by resetting capillaries of different lengths. On this occasion, an analyst must set a multiple of capillaries one after another in the electrophoretic instrument, posing the problem of much expense in time and effort.
Further, in this method, in order to load a sample into the capillaries, cylindrical electrodes are beforehand brought into contact with the sample in the sample plate, and then by driving and moving the sample plate, the above capillaries are inserted into the cylindrical electrodes, thereby to bring the capillaries into contact with the sample. Therefore, in order to simultaneously insert a plurality of capillaries with an outer diameter of several hundreds of micrometers into cylindrical electrodes, extremely high accuracy must be required of a driving portion of the sample plate or the cylindrical electrodes must have an inner diameter with a sufficient allowance.
When accuracy is given to the above driving portion by reducing the diameter of the cylinder of above electrode, a sample measured last time remains in a gap between the inner surface of the cylindrical electrode and the outer surface of the capillary due to the capillary phenomenon, posing the problem that a good-accuracy electrophoretic analysis is impossible.
When accuracy is not required of the driving portion by increasing the diameter of the cylinder of the electrode, the possibility of mixing of other samples due to the above capillary phenomenon decreases. However, this case poses the problem that because of the large diameter of the electrode, the bottom end of the electrode does not reach the well bottom of the sample plate with such an inverted cone shape that the well becomes narrower toward the bottom.
Because the bottom end of above electrode does not reach the well bottom, it does not come into contact with a sample or a buffer solution, with the result that in principle, sample loading and electrophoresis are impossible. Therefore, in a case where the electrode is to be brought into contact with the above sample and buffer solution and a general-purpose microtiter-plate is to be used, a minimum amount of sample must be set at a large value in order to raise the liquid level of the sample and buffer solution, thus posing another problem.